This invention relates to a spiral scanning type X-ray CT apparatus.
The spiral scanning type X-ray CT apparatus is known in the art as described, for example, in U.S. Pat. No. 4,789,929.
FIG. 2 of the accompanying drawings illustrates a schematic construction of the spiral scanning type X-ray CT apparatus according to the prior art. A CT scanner 3 is a spiral scanning type scanner and comprises a bed for supporting a subject, a gantry for allowing this bed to be loaded and pulled out and an X-ray source (inclusive of members such as a collimator, etc.) rotating around the gantry so that the X-ray source can be revolved while moving the bed, and the subject can be scanned spirally.
A CT controller 2 executes processing and control. The term "control" hereby means the control of movement and rotation of the CT scanner 3 and the timing control of X-ray irradiation. The term "processing" means input and storage processing of spiral data measured by the CT scanner 3, reconstruction processing of the spiral data and display processing of the reconstructed image on a display 4.
According to the CT apparatus of the prior art described above, an operator of the apparatus must input three imaging conditions, that is, the moving speed of the bed, the irradiation slice width of the X-ray beams and the pitch of the image, from an input unit 1.
In a non-spiral scanning type X-ray CT apparatus wherein the bed is kept in a halted condition halt at the time of imaging, the irradiation slice width and the image pitch are set as the imaging condition.
Here, let's take the example of mass screening with the X-ray CT apparatus. This mass screening is directed to sequentially apply CT scan to a large number of people (subjects) and to discover diseases in early stages. In other words, quick measurement of a large number of subjects by CT scan and appropriate diagnosis from the measurement result become necessary.
In such mass screening, a spiral scan capable of effecting a CT scan with a small exposure of the whole body of people is suitable for use. For, in the case of a spiral scan, the whole body is moved inside the gantry and while the X-ray source is rotated by 180.degree., 360.degree. and further, 720.degree., during this movement, the X-ray beams are exposed. Interpolation processing, etc., is applied to the spiral data obtained by this spiral scan, and a reconstructed image for each slice vertical to the body axis (that is, a tomogram ) is obtained.
However, when the X-ray beams of 180.degree., 360.degree. and further, 720.degree., are exposed in the same measuring condition to the whole body, the reconstructed images thus obtained does not fully meet the diagnostic purpose, therefore a spiral scan is generally carried out for each diagnostic portion of the body. The term "diagnostic portion" of the body means each characterizing portion of the body such as the head, the chest (lung), the liver, and so forth. Furthermore, depending upon the object of diagnosis, zones in the direction of the body axis of spiral scan are also different. In any case, a spiral scan capable of obtaining the tomograms of a greater number of slice portions with smaller exposure than the conventional stationary bed type CT scan, in which the X-ray source is rotated 180.degree. or 360.degree. while the bed is fixed, is much more preferred in the mass screening.
When a spiral scan is effected for each diagnostic portion of the body, the moving speed of the bed is mostly changed for each diagnostic portion so as to improve measurement efficiency and reliability. When the chest is diagnosed, for example, the bed is moved at a high speed while the bed is moved at a low speed to diagnose the head. On the other hand, the number of tomograms to be obtained becomes the problem depending on the diagnostic portions. When more precise diagnosis is necessary, a greater number of tomograms are required. The number of necessary tomograms varies, too, depending on the size of the width as viewed from the direction of the body axis of spiral scan. The number is determined by the pitch for obtaining the tomograms. The greater the pitch, the smaller becomes the number, and the smaller the pitch, the greater becomes the number, on the contrary.
Further, the irradiation slice width of the X-ray beams affects image quality of the resulting tomogram. To obtain higher image quality, the slice width must be reduced.
In the X-ray CT apparatus according to the prior art, all of the bed moving speed V, the pitch P and the slice width D as the imaging conditions must be inputted from the input unit 1.
Each of these imaging conditions assume various values depending on the experience and the object of diagnosis. If such imaging conditions are set at each time for the diagnostic portions and for the object of diagnosis, quick CT measurement cannot be made in CT measurement of a large number of people such as a mass screening.